Optical communication systems are enhanced through increases in spectral efficiency (SE) to their transmitted signals. The use of orthogonal polarization states to transmit data signals in parallel, referred to as polarization multiplexed (PM) signaling, offers a valuable doubling of SE. To implement a PM system, an effective method of separating, or demultiplexing, the two PM signals at the receiver side is necessary. A most common approach for PM signal demultiplexing is to track the optical states of polarization (SOP) and rotate them via a polarization controller (PC) to align with the principal states of a polarization beam splitter (PBS). After the PBS each signal will propagate alone to an independent photoreceiver channel. Hindering the demultiplexing operation at the receiver are physical impairments, which reduce the orthogonality of the transmitted PM signals. With such polarization impairments, the optical tracking method cannot simultaneously align both PM signals with the principal axes of the PBS. In accordance, the PM signals are not separated perfectly, leading to PM signal crosstalk. An advanced method for isolation of non-orthogonal PM signals is required.
At present, non-orthogonal PM signal demultiplexing has been shown possible only through digital signal processing (DSP) of coherently received optical signals. In these systems digital manipulation of received signals allows for the isolation of PM signals that are not perfectly separated through physical mechanisms. These demonstrations are limited by the speed and resolution of available digitization and DSP hardware.
FIG. 1 shows a prior art optical tracking approach which utilizes an automated PC 2 that is controlled through a feedback mechanism. In this prior approach, the two outputs of the PBS 4 are directed to demultiplexed receiving channels. The system relies on the assumption of S1 and S2 orthogonality for proper signal isolation, because the PBS 4 utilizes an orthogonal basis for its output channels. However, S1 and S2 are not always orthogonal, due to impairments, thus, at a minimum, S1 or S2 is corrupted by crosstalk at its respective output.